There are several prior patents and publications which are directed to the preparation of cobalt oxide optically black surface coatings. All of these are concerned primarily with the manufacture of solar collectors which are highly absorptive or opaque to light in the visible region, but reflective or transparent to light in the infrared region to provide a low emissivity. The purpose in a solar collector is to capture as much of the solar energy as possible accompanied by removal of the energy through thermal conductivity and to minimize the loss of energy through reradiation.
A particular problem addressed in many of the prior patents and publications is the development of coatings which are stable at the high temperatures experienced in solar collectors, for instance, 600.degree. C. It has been the experience in the art that coatings, including cobalt oxide coatings, tend to degrade at high temperatures causing a reduction in absorptivity.
In the publication "Preliminary Study of a Solar Selective Coating System Using Black Cobalt Oxide for High Temperature Solar Collectors", G. McDonald, American Vacuum Society, International Conference on Metallurgical Coatings, April 21-25, 1980, Paper No. 14, it was reported that improved cobalt oxide films were obtained by electrodedeposition from a strongly oxidizing solution. The oxidizing agent in the cobalt bath was hydrogen peroxide. The films were said to give an absorptance/emittance of 0.90/0.20. These properties were determined to be stable for exposures up to at least 1,000 hrs at 650.degree. C. It was also reported in the McDonald paper that these properties could be obtained by thermal decomposition of cobalt nitrates to form an absorber layer of mixed as well as simple cobalt oxides. Details of the thermal decomposition process were not disclosed in the paper.
This disclosure is also the subject of issued U.S. Pat. No. No. 4,392,920, by Glen E. McDonald, dated Jul. 12, 1983.
Also of interest with regard to the preparation of black cobalt oxide coatings is the publication "Solar Selective Black Cobalt: Preparation, Structure, and Thermal Stability", Smith et al., J. Appl. Phys., 51(8), August, 1980, 4186-4196, which discusses the preparation of cobalt oxide black coatings by thermal oxidization of elctroplated cobalt metal at 400.degree. C. With this coating, optical degradation was observed at 500.degree. C. It was speculated, in the article, that the thermal change may have been due to loss of hydroxyl groups. It was also reported in this paper that thinner films with smaller emittances and lower absorptance tended to be more prone to degradation.
A paper entitled "Selective Radiation Coatings. Preparation and High Temperature Stability", Kokoropoulus et al., Solar Energy, Vol. 3(4), 1959, pages 19-23, appears to suggest the method of preparing cobalt oxide (Co.sub.3 O.sub.4) coatings by electroplating cobalt onto a polished reflecting metallic surface and oxidizing the cobalt by heating it in air (page 19, col. 1, penultimate paragraph). One coating on a silver substrate was said to give an absorptivity of 0.90. No details are given concerning the preparation method in general, or that used specifically in preparing the sample said to have an absorptivity of 0.90. It was reported that the cobalt oxide coatings tended to be thicker and gave slightly higher emissivities than cupric oxide. The coatings were subjected in test procedures to high temperatures for prolonged periods, and were reported to have good heat stability. After twelve hours at 900.degree. C., the Co.sub.3 O.sub.4 on silver was reported to be in excellent condition. Cobalt oxide on platinum was heated at 1100.degree. C. for 26 hours and was said to show no alloying nor loss of blackness.
Prior U.S. Pat. No. 4,473,447 to Herbert Gutwein et al. discloses the manufacture of solar absorptive layers by electrodeposition of heavy metals such as nickel and cobalt from an aqueous nickel or cobalt citrate solution. The layers deposited by electrodeposition were said to have high absorption in the visible spectral range and a high resistance to temperature, although no specific temperature limit was given in this patent.